1. Field of the Invention
The present invention relates to a color filter, and more particularly to a color filter for a transflective liquid crystal display.
2. Description of the Related Art
Liquid crystal displays (LCDs) are divided into three types: transmissive LCDs, reflective LCDs, and transflective LCDs. The transmissive LCD displays show images using light from a back light device. Only about 10% of light generated by the back light is utilized after passing through polarizers and LCD panel. Therefore, the transmissive LCD employs a backlight device capable of high brightness, requiring high power consumption. The reflective LCD uses ambient light to display images, thus reducing power consumption. The reflective LCD, however, can only be used during the day or in an office where external light is present, but not under dim lighting conditions.
Therefore, transflective LCDs have been introduced. FIG. 1 is a cross-section of a conventional transflective liquid crystal display. The transflective LCD includes opposing upper and lower substrates 160 and 150, a liquid crystal layer 180 interposed therebetween, and a backlight 170 under the lower substrate 150. A common electrode 162 is formed between the upper substrate 160 and the liquid crystal layer 180. A transmissive electrode 164 is formed in the transmissive region t of the lower substrate 150. A reflective electrode 152 is formed in the reflective region r of the lower substrate 150. A color filter layer 168 is interposed between the upper substrate 160 and common electrode 162. In transmissive mode, light 174 emitted from the backlight 170 passes through the lower substrate 150, the transmissive electrode 164, the color filter layer 168, and the upper substrate 160. In reflective mode, ambient light 172 passes through the upper substrate 160 and color filter layer 168, is incident to the reflective electrode 152, is reflected by the reflective electrode 152, and passes through the color filter layer 168 and the upper substrate 160 again.
As mentioned above, in the transmissive region t, light 174 emitted from the backlight 170 passes through the color filter layer 168 only once. In the reflective region r, however, ambient light 172 passes through the color filter 168 twice. Consequently, the color saturation in the reflective region will be higher than that in the transmissive region.
In order to solve the above problem, Tomohisa Matsushita et al. in U.S. Pat. No. 6,501,521 disclose forming a hole or slit in the color resist in the reflective region and then filling a transparent material therein, thus changing the color saturation in the reflective region. For example, FIG. 2 is a top view of color resists in a pixel region of a transflective liquid crystal display. The pixel includes R (red), G (green), and B (blue) subpixel regions. Each subpixel region includes reflective and transmissive regions. The reflective regions are labeled R(r), G(r) and B(r), the transmissive regions are labeled R(t), G(t) and B(t), and the color resists are labeled 210R, 220G, and 230B. The red resist 210R in the reflective region R(r) in the R subpixel region is partially removed and then filled with a transparent material 210W. The green resist 220G in the reflective region G(r) in the G subpixel region is partially removed and then filled with a transparent material 220W. The blue resist 230B in the reflective region B(r) in the B subpixel region is partially removed and then filled with a transparent material 230W. Color mixing of the transparent and color resists in the reflective region can be controlled by adjusting the size of the opening, in order to decrease the color saturation in the reflective region. Thus, the color saturation in the reflective and transmissive regions becomes substantially equal.
The above method can decrease the color saturation in the reflective region. However, referring to FIG. 3, the color saturation (NTSC(%)) in the reflective region is limited when brightness is low (Y<15.0) and differs greatly from the color saturation in the transmissive region.
A resist direct electrodeposition method is used so as to produce a color filter with different color purity between the transmission regions and the reflection regions. With this method, color filter portions with different color purity are formed in the transmission regions and in the reflection regions for each color. In this way, it is possible to increase the color saturation in a transmission mode while maintaining the brightness in a reflection mode. However, forming color filter portions of different transmittances in the transmission regions and in the reflection regions increases the number of steps in the color filter electrodeposition process. Specifically, electrodepositing reflection color filter portions and transmission color filter portions for each of R, G and B requires a total of six photolithography steps. Moreover, it requires two color filter materials of different color purity for each color. Thus, the production cost increases.